In general, a polyimide (hereinafter referred to as "PI") resin denotes a high heat-resistant resin which is manufactured by condensation polymerization of aromatic tetracarboxylic acid or the derivatives thereof with aromatic diamine, or aromatic diisocyanate followed by imidization. However, the PI resin is insoluble in solvent and non-fusible by heat.
Further, the PI resin can have a variety of molecular structures depending on the types of the monomers thereof. As for the aromatic tetracarboxylic acid constituent, pyromellitic dianhydride (PMDA) or biphenyltetracarboxylic dianhydride (BPDA) are used herein. As for the aromatic diamine constituent, oxydianiline (ODA) or p-phenylene diamine (p-PDA) are used for condensation polymerization. The typical PI resin has a repetitive unit of the following formula 10: ##STR7##
The PI resin with the formula 10 as repetitive unit is high heat-resistant, insoluble, and non-fusible with the following properties:
(1) excellent thermal oxidative property; PA1 (2) superior heat-resistance in terms of using temperature, i.e., 260.degree. C. for a long-term use, and 480.degree. C. for a short-term use; PA1 (3) excellent electric and mechanical properties PA1 (4) excellent radiation resistance and low temperature properties; PA1 (5) intrinsic non-combustible properties; and, PA1 (6) excellent chemical-resistant properties.
In spite of the fact that the PI resin with formula 10 as repetitive unit possesses an excellent heat resistance property, the processing therein is extremely difficult due to insolubility and non-melting property.
To overcome the shortcomings of the PI resin, several methods have been devised: a) method of introducing polar groups into the backbone or side chains of the polymer, b) method of introducing the connecting groups or pendant groups with a large volume into the polymer, and c) method of enhancing the flexibility of the backbone of the polymer.
In particular, as part of research to enhance the solubility of the PI resin, T. Kurosaki et al. reported a method of preparing a soluble PI resin using alicyclic anhydride as a monomer (Macromolecules, 1994, 27, 1117 and 1993, 26, 4961) Also, Qn Jin et al. disclosed in 1993 a method of preparing a soluble PI resin using the cyclic diamine (J.P.S. Part A. Polym. Chem. Ed., 31, 2345.about.2351).
The soluble PI resins as modified by the aforementioned methods have improved the solubility due to enhanced flexibility of the chains. However, the actual use thereof seems to be problematic since the high thermal stability and mechanical characteristics, which are advantages of the conventional PI resin, are seriously affected thereby.
And the soluble PI resin (Matrimid 5218) made by Ciba Geigy Co. showed excellent heat-resistance and solubility by introducing trimethylindane group into the main chain. However, the actual use thereof seems go be problematic since monomers containing trimethylindane group must be prepared through many steps.
To overcome such shortcomings, the inventors herein have for several years conducted intensive studies to improve the solubility and melting property of the polyamideimide resin, a typical thermoplastic and high heat-resistance resin, for improvement in the processability of the conventional aromatic high heat-resistance polymers. As a result, the inventors have discovered that the solubility of a polymer can be significantly enhanced by introducing isophorone diamine (hereinafter referred to as "IPDA"), an aliphatic diamine compound containing trimethylcyclohexyl group, into the backbone of polymer (U.S. Pat. No. 5,521,276).
Further, the inventors have endeavored to prepare a soluble PI resin containing IPDA moiety with better heat resistance than the conventional PI resin. As a result, the inventors have succeeded in preparing PI resin with enhanced heat resistance and solubility using aromatic diamine compounds containing cyclohexylidene group with various structures of substituted alkyl groups as the monomers (Korean Patent Application No. 97-2811 and Korean Patent Application No. 97-21577).
Based on the above studies, the inventors have significantly increased the solubility and heat resistance of a polymer by introducing polyalicyclic structures instead of alkyl group substituted cyclohexylidene group, which is a connecting group between the two phenyl groups.